1. Field of the Invention
The present invention relates to a substrate providing a ferroelectric crystal thin film, its production method and a device using said substrate. In particular, it relates to the substrate providing a ferroelectric crystal thin film used for a non-volatile memory device of ferroelectrics, a pyroelectric infrared sensor, a piezoelectric device and the like, its production method and a device using the substrate.
2. Description of the Related Art
Since a ferroelectric crystal has a lot of characteristics such as spontaneous poralization, high dielectric constant, electro-optic effect, piezoelectric effect, pyroelectric effect and the like, it is applied for many kinds of devices such as a capacitor, oscillator, optical modulator, infrared sensor and the like. In case of application for these devices, the ferroelectric crystal has been used in the form of a single crystal or ceramics. However, in accordance with a development of high quality ferroelectric thin film prepared by various methods such as vacuum evaporation, sputtering, CVD (Chemical Vapor Deposition) method, sol-gel method and the like, an application different from the conventional cases using the bulk material has been expected.
Recently, it was found that a ferroelectric non-volatile random access memory (FNVRAM) can be obtained with high density and high speed by a combination of the ferroelectric crystal with a semiconductor memory such as DRAM, and the study for such a device has been proceeding.
It is necessary for the development of the device to provide a high quality thin film in which remanent spontaneous polarization (Pr) is great, coercive field (Ec) is small, a leak electric current is low, and a film thickness is 200 nm or less for reducing a driving voltage and for simplifying a manufacturing process.
Especially, intensive study for materials has been done on PZT (Pb(Zr.sub.1-x Ti.sub.x)O.sub.3) with which an excellent ferroelectric thin film can be obtained by a sputtering method and the like. However, in spite the fact that the ferroelectric characteristics of PZT greatly depends on the composition x, the composition easily varies in a step of film formation or thermal treatment because it contains Pb having a high vapor pressure. As a result, a pin hole or uneven surface readily occurs, which causes a leak current or switching fatigue when the film thickness decreases. Accordingly, an attention was drawn to a high quality thin film comprising Bi.sub.4 Ti.sub.3 O.sub.12 as a material which has a ferroelectric characteristics similar to PZT and does not contain a substance having a high vapor pressure such as Pb.
The thin film of Bi.sub.4 Ti.sub.3 O.sub.12 can be made by a sol-gel method or MOCVD method. However, a thin film conventionally manufactured so far has a difficulty in obtaining a densed film because it comprises crystal particles with a large size and the surface of the film is significantly uneven. Accordingly, leak current increases in accordance with reduced film thickness, so that sufficient ferroelectric characteristics can not be realized.